In recent years, it has become increasing common for human users to use graphical user interfaces ("GUIs") to operate general-purpose computer systems. Such modern graphical user interfaces utilize a concept called "desktop metaphor," in which "objects" of all types are represented graphically as pictorial icons. Many kinds objects may exist within a computer system. Some kinds of objects, such as document objects, contain data. Other objects include programs, locations within intra-machine or intermachine file systems, or representations of peripheral devices connected to the computer system.
According to a user input technique called "drag and drop," GUI users may specify a data transfer by using a pointing input device such as a mouse to "drag" an icon representing an object containing data onto an icon representing an object that may receive data and "drop" it there. The object that is dragged is called the "source object." The object upon which the source object is dropped is called the "destination object." As a result of the drag and drop operation, a default data-receiving method of the destination object is invoked upon the source object. Thus, by dropping a source object on a program object, a user may open the source object for editing using that program object; by dropping it on a file system location object, the user can cause a copy of the source object to be created at the file system location associated with the file system location object; by dropping it on a peripheral device object, a user may perform a common function associated with the peripheral device on the source object, e.g., by dropping the source object on a printer object, the user may print the source object on the printer associated with the printer object, while, by dropping the source object on a modem object, the user may transmit the source object to another computer system using the modem associated with the modem object. Depending on the identities of the source and destination object, and any preference expressed by the user after the drag and drop, the source object may continue to exist independent of the destination object (in which case the drag and drop operation is characterized as a "copy") or the source object may be deleted (in which case the drag and drop operation is characterized as a "move"). The specific handling of drag and drop operations is determined in each case by the destination object.
While drag and drop operations perform an important function within GUIs that support them, they can be difficult for users to carry out. First, a user must ensure that the icons representing the source and destination objects are both visible. In each case, this may involve traversing an extensive hierarchy of all of the objects within the computer system, a potentially time-consuming process that is exacerbated when the objects' locations in the hierarchy are unknown. Second, the user must drag the source object onto the destination object and drop it there, a process that requires a fair amount of dexterity. In order to perform the drag and drop, the user must depress a button on the pointing device while the pointing device is pointing at the icon representing the source object, then hold the button down while moving the pointing device to point to the destination object, then release the button while the pointing device is pointing at the destination object. In practice, while it is relatively straightforward for a user to move the pointing device to point at the source object and depress the button, it is difficult, especially for novice users, to move the positioning device with the button held down and release the button only when the pointing device is pointing to the destination object.